Custom functions in FileMaker: when to create them and recursion

Expert

Custom functions extend FileMaker's calculation language with reusable, named functions. Learn when a custom function is the right abstraction, how to handle recursion limits, and best practices.

What you'll learn

When a custom function is the right abstraction vs a Let() or a script
How to write a recursive custom function with a base case and depth limit
Practical custom function patterns: list processing, string formatting, business rules

Custom functions are reusable calculation routines with named parameters that appear alongside built-in functions — ideal for business logic used in multiple places, and for recursive algorithms on text lists.

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Create a simple reusable custom function

Define custom functions in File > Manage > Custom Functions. Give it a name, declare parameters, and write the calculation body. It then appears in the calculation editor like any built-in function.

FileMaker Script

// Custom function: FormatCurrency ( amount )
// Returns a formatted dollar string, e.g. "$1,234.56"

"$" & Text ( amount ; "#,##0.00" )

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